Re: [Qemu-devel] [RFC 00/28] s390x CPU models: exposing features

[Eduardo Habkost]

On Tue, Jun 21, 2016 at 10:56:28PM +0200, Jiri Denemark wrote:
[...]
> Let me sum up what libvirt is doing (or will be doing) with guest
> CPUs... Libvirt supports three guest CPU configuration modes:
> 
> - host-passthrough -- this is easy, it's just asking for "-cpu host" and
>   no fancy commands are required.
> 
> - host-model -- for this we need to know what CPU configuration we need
>   to ask for to get as close to the host CPU as possible while being
>   able to ask for the exact same CPU on a different host (after
>   migration). And we need to be able to ask for this at probing time
>   (when libvirtd starts rather than when starting a new domain) using
>   "-machine none,accel=kvm:tcg", that is without actually creating any
>   guest CPU. This is what Eduardo's query-host-cpu QMP command is useful
>   for. In x86 world we could just query the guest CPU after running QEMU
>   with "-cpu host", but that's unusable with "-machine none", which is
>   why another way of getting this info is needed.

Question: KVM supports x2apic (and enables it by default) on any
host CPU, because it is all emulated by KVM. Should "host-model"
include x2apic on all hosts, or only when the host CPU has it?
("-cpu host" does include it).

Including x2apic sounds more useful, but it doesn't match the
"get as close to the host CPU as possible" part of your
description.

-- 
Eduardo